Method for preparation of carbonitride nuclear fuel materials

ABSTRACT

A RAPID METHOD FOR CONVERTING ACTIMIDE OXIDE-CARBON PARTICLES TO ACTINIDE CARBONITRIDE IS GIVEN AND COMPRISES CONTACTING THE ACTIMIDE OXIDE-CARBON PARTICLES WITH NITROGEN IN A FLUIDIZED BED FURNACE AT ELEVATED TEMPERATURS.

United States Patent M 3,577,485 METHOD FOR PREPARATION OF CARBONITRIDENUCLEAR FUEL MATERIALS Ronald L. Beatty, Oak Ridge, James M. Leitnaker,Kingston, and Karl J. Notz, Jr., Oak Ridge, Tenn., assignors to theUnited States of America as represented by the United States AtomicEnergy Commission N0 Drawing. Filed Feb. 3, 1969, Ser. No. 796,175

Int. Cl. G21c 21/00 US. Cl. 264.5 7 Claims ABSTRACT OF THE DISCLOSURE Arapid method for converting actinide oxide-carbon particles to actinidecarbonitride is given and comprises contacting the actinide oxide-carbonparticles with nitrogen in a fluidized bed furnace at elevatedtemperatures.

BACKGROUND OF THE INVENTION The invention described herewith was made inthe course of, or under, a contract with the U.S. Atomic EnergyCommission. It relates generally to nuclear reactor fuels and moreparticularly to a method for preparing actinide carbonitride nuclearfuel materials.

Of the various actinide compounds investigated as candidate fuels fornuclear reactors uranium dioxide probably has been most widelydemonstrated. In a fast breeder nuclear reactor a fuel which has ahigher fuel density and higher thermal conductivity than uranium dioxideis necessary. As a candidate fast breeder reactor fuel uraniummonocarbide and uranium mononitride are of interest because of theirhigh uranium density and high thermal conductivity. Both uraniummonocarbide and uranium mononitride have certain unfavorable propertieswhich detract from their attractiveness as nuclear fuels, the formerbecause of its reactivity with metal cladding and the latter because ofits high cost. In a United Kingdom Atomic Energy Authority Memorandum(AERE-M-1360, Variation of Some Thermodynamic Properties Across theUC-UN Solid Solution Range, M. H. Rand, 1964), it has been proposed thata mixed fuel of uranium monocarbideuranium mononitride, i.e., uraniumcarbonitride, as a solid solution would be better as a compromise fuelcandidate than either uranium monocarbide or uranium mononitride itself.Moreover, in order to reduce the fuel cycle cost of a liquid metalcooled fast breeder reactor (LMFBR), it may be desirable to utilize acarbonitride fuel material rather than pure carbide or pure nitride.

Previously, uranium carbonitride has been prepared by several reactions.One method has been to react uranium monocarbide 'with nitrogen at anelevated temperature. Another method has been to react uranium dioxidewith carbon to form uranium monocarbide and in a second step react theuranium monocarbide with nitrogen. A third method is to react uraniummononitride with graphite powder. A still further method has been toreact a uranium oxide-carbon mixture in the presence of nitrogen. Ineach of the methods the reactants are mixed at room temperature andheated to the reaction temperature at a slow and controlled rate. Onoccasion a thermal cycling step must be utilized during conversion inorder to help penetrate a thin impermeable skin which forms around theparticles. Each of the aforestated processes are laborious andtime-consuming, requiring several hours to accomplish. It is thereforehighly desirable and an object of this invention to provide a method forpreparing actinide carbonitride nuclear fuel materials without thehereinbefore mentioned drawbacks of the prior art processes.

3,577,485 Patented May 4, 1971 SUMMARY OF THE INVENTION Briefly, thepresent invention comprises the steps of contacting actinideoxide-carbon particles in a fluidized bed furnace maintained at anelevated temperature (l4001900 C.) with a fiuidizing gas of nitrogen andwithdrawing the formed actinide carbonitride product. Contrary to thegeneral teaching of the prior art that a slow heat-up cycle is requiredin the preparation of uranium carbonitride, applicants have found thatessentially complete conversion of a uranium dioxide-carbon gel mixtureto a uranium carbonitride product could be rapidly achieved by'droppingthe starting mixture into a hot zone of a vertical tube furnace andfluidizing with nitrogen. UO -CfllbOIl sol-gel particles were convertedin about 5 to 15 minutes to uranium carbonitride in accordance with theprocess steps of this invention. Where the uranium carbonitride washeated an additional 15 minutes in argon single phase material wasformed.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The method of this invention isbroadly applicable to converting actinide oxide-carbon particles intoactinide carbonitride fuel materials. The actinide oxide-carbonparticles which include UO C, ThO -C, PuO C, CmO C, etc., and mixturesthereof have been previously prepared by a number of different methods.In one method disclosed in US. Pat. 3,290,122 UO -carbon gelmicrospheres are prepared by adding finely divided (mean particle sizenot exceeding 400 angstroms), such as channel blac carbon, to the U0starting sol and blending the resulting mixture to form a homogeneousdispersion and thereafter passing the mixed UO -carbon sol into asphereforming column to form sol droplets which are subsequentlydehydrated and dried to green gel microspheres. Similarly, UO carbonshards may be prepared by placing the UO -carbon sol in drying pansinstead of passing it into the column and evaporating the sol to drynessat a temperature in the range of -135 C., whereupon the dried gelmaterial fractures into shards.

As has been noted a serious difficulty with the slow heat-up prior artprocesses was the formation of a sintered or impervious skin about theUO -carbon particles which prevented reaction of the nitrogen with theinner portion of the particle. While applicants do not wish to be boundby a rigid theory of their invention, it is postulated that the reactionconverting the oxide-to-carbide-to-carbonitride goes to completionbefore sintering of the mixture can occur. The rapid conversion appearsto be aided by the presence of the finely divided carbon which providesa path for the nitrogen gas to diffuse into the core of the particles tofacilitate essentially complete conversion of the fuel particles.Suflicient carbon should be provided in the starting UO -C gel mixtureso as to provide for free carbon, i.e., that not required to form U(C,N) in the reacted particles. This is seen from the fact that where nofree carbon is initially present in uranium dicarbide, carbon producedby reaction of the UC with nitrogen formed a symmetrical shell on theparticle as the reaction proceeded and the reaction was slower by afactor of 2 to 5 than when free carbon was present initially. It shouldthus be apparent that this rapid conversion, which for UO -carbon gelmixtures is essentially complete in about 15 minutes at 1500 C., iscontrary to what would be expected from the prior art processes forconverting metal oxide-carbon gel materials to correspondingcarbonitrides which required several hours.

The method of the invention may be carried out in fluidized bed furnacesof conventional design. One such embodiment comprises a 1-inch diameterconical bottom graphite fluidizing chamber heated in a graphiteresistance furnace. The fiuidizing gas is nitrogen supplied at a flowrate of 1 liter/minute. In accordance with this method fluidization isachieved by passing nitrogen upflow through the fluidized bed furnace.The temperature of the furnace is next equilibrated to the desiredreaction temperature. The UO -carbon gel mixture is then droppeddirectly into the hot zone of the fluidized bed furnace and fluidizedfor a period of time to convert the U to uranium carbonitride. After theconversion is completed the uranium carbonitride product should beprotected from air oxidation, such as by coating the particles withcarbon or cooling under a non-oxidizing atmosphere.

It has been noted that excess carbon is desirable to increase the UO Creaction rate. The free carbon may be subsequently removed by heatingthe formed product in nitrogen at a reduced pressure or changing theflow gas to argon. This results in a single phase uranium carbonitride.

Examination of the formed products revealed them to be uraniumcarboniride containing only minor amounts of oxygen. Where an inert gastreatment was given subsequent to the conversion step, the product wassingle phase. Uranium carbonitride products formed from U0 carbon gelshards are readily cold-pressed and sintered into fuel compacts havingdensities up to about 85% of theoretical.

Further illustration of the quantitative aspects and procedures of thepresent invention are given in the following examples.

Example I The feasibility of converting UO -carbon gel mixture in theform of microspheres into uranium carbonitride microspheres wasestablished employing the apparatus hereinbefore described as follows:about 4 grams of U0 carbon microspheres (140,) having a density of 1.73g./cc. were dropped directly into the hot zone of the fluidized bedfurnace which was maintained at a temperature of 1500 C. Nitrogen wasflowing through the furnace at the rate of about 1 liter/minute. Thecarbonto-uranium ratio for the mixture was 10.

After 15 minutes at temperature the microspheres were coated with carbonto preclude oxidation during subsequent analysis. Methane was mixed withthe nitrogen and the microspheres fluidized at 1500 C. for about 5minutes. Then the methane flow was shut off and the nitrogen flowcontinued to purge the system. The product was analyzed by X-raydiffraction and revealed the product to be uranium carbonitride plusgraphite. The lattice constant for the product was 4.9011 which liesbetween the lattice constant of uranium mononitride (4.8892) and thatfor uranium monocarbide (4.960 2) which values were derived at Oak RidgeNational Laboratory in earlier experiments.

Example II In another run, a 4.7 gram charge of UO -carbon gel shardswas dropped directly into the hot zone of the fluidized bed furnace inthe presence of flowing nitrogen. The furnace temperature was maintainedat 1700 C. for

15 minutes and the nitrogen feed rate 1 liter/minute. The

carbon-to-uranium ratio was 2.4 and the particle size of theoxide-carbon mixture was l00, +200 mesh.

After 15 minutes the gas was changed .to argon and held at 1700 C. foranother 15 minutes. Subsequent analysis revealed a lattice constant of4.9043 (composition-0.821 mole fraction uranium mononitride and 0.179mole fraction uranium monocarbide) and an oxygen content of 0.3%.Pressing and sintering reduced the oxygen content to 0.2%. A sintereddensity of 12.11 g./cc. theoretical) resulted.

It will be appreciated that the experiments described in the aboveexamples were not performed with the thought in mind of optimizingprocess parameters and procedures but rather were performed merely toshow operability of the process. Hence, slight variations in the processparameters heretofore mentioned may be made but still be within thescope of applicants invention; namely, to achieve a rapid conversion ofactinide oxidecarbon mixtures to actinide carbonitrides in a fluidizedsystem at temperatures in the range of 1500 C. employing nitrogen as thefluidizing and reaction gas. For example, a mixed heavy metaloxide-carbon mixture such as (UO PuO )C, (ThO UO )C, and

may be employed to prepare the corresponding mixed actinidecarbonitride.

It will be understood that the invention is not intended to be limitedto the specific embodiments given by way of illustration of the processbut that it may be modified within the scope of the appended claims.

What is claimed is:

1. A fast process for preparing actinide carbonitride particlescomprising the steps of contacting actinide oxidecarbon particles in afluidized bed furnace maintained at a temperature in the range of1400l900 C. with a fluidizing gas of nitrogen and thereafter withdrawingthe formed actinide carbonitride.

2. The process of claim 1 wherein said actinide oxidecarbon particlescomprises ThO C, UO C, PuO C and mixtures thereof.

3. The process of claim 1 wherein said actinide oxidecarbon particlescomprise UO C gel mircopheres.

4. The process of claim 1 wherein said actinide oxidecarbon particlescomprises UOz-C gel shards having a mesh size of to +200 and +50.

5. The process of claim 3 wherein said UO C gel microspheres have acarbon to uranium ratio of 10.

6. The process of claim 4 wherein said UO C gel shards have a carbon touranium ratio of 2.4.

7. The process of claim 1 wherein said nitrogen feed rate is 1liter/minute.

References Cited UNITED STATES PATENTS 3,306,957 2/1967 McLaren 264--0.5

3,309,322 3/1967 Anselin et a1. 2640.5X

3,368,877 2/1968 Guyton et al 264O.5X

FOREIGN PATENTS 1,437,046 3/1966 France 2640.5

BENJAMIN R. PADGETT, Primary Examiner S. HELLMAN, Assistant Examiner

